The invention concerns a rubber article composed of at least two vulcanized mixes having different composition and properties, and having between them a lap joint. More particularly, it concerns a tire whose various constituent mixes are joined by superposition of an edge of one of the mixes over another mix.
Junctions between mixes such as those mentioned above, when subjected to stresses (whether tension, compression or shear), represent a particularly vulnerable area of the article considered, the life of the article being greatly limited by the destruction of the joint, whether this destruction be due to adhesion failure or to stress concentration at the location of the joint, or even to external aggression in the case of some junctions.
The purpose of the invention is to improve the life of the article considered by causing the junction(s) between rubber mixes constituting the said article to be less influenced by the known causes of its/their destruction.
The vulcanized rubber article according to the invention, which consists of at least two rubber mixes having different composition and properties, the said two mixes having between them a lap joint, is characterized in that at least one of the edges of at least one of the two mixes has an end whose trace-line resembles an oscillating movement, namely an oscillating trace-line.
The thickness of the edge of the mix in question is preferably constant over a width at least equal to the desired trace-line amplitude: the said thickness being in any case less than 2 mm in the non-vulcanized condition such that the molding and vulcanization of the finished article obliterates the surface irregularities created at the junction of the two mixes.
Any trace-line may be suitable (trace-lines of stationary or non-stationary oscillations), the preferred trace-lines being the sinusoidal trace-line of a harmonic oscillation and the circular trace-line (in which the line corresponding to a half-period is a semicircle).
The preferred application of the crenellated or serrated edge(s) relates to the joining of two rubber mixes constituting a tire, whether this junction emerges on an external wall of the tire or is totally internal. The application is particularly advantageous for junctions on the sidewall of the tire between the mix used for the tread and that used for the sidewall.
An oscillatory trace-line can be characterized by an amplitude and a wavelength. The amplitude measured crest to crest, whether variable or not, is in the case of tire applications preferably between 3 mm and 15 mm. As for the wavelength, this is preferably between 0.1% and 2% of the circumferential extension of the tire measured in the equatorial plane.